Status and Prognosis of Future-Generation Photoconversion to Photovoltaics and Solar Fuels
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rofessor Arthur J. Nozik has fought for, inspired, cajoled, and led a generation of scientists in the pursuit of the science of solar photoconversion, photovoltaics, and solar fuels. On March 25th, 2016, a group of former colleagues, co-workers, and friends met to recognize Prof. Nozik’s contribution to their work, excellence in science, and life. While the event was a celebration of his many scientific contributions, it served mostly to honor his leadership and vision. Early in Prof. Nozik’s career he knew that he not only wanted to do high-quality science, he wanted to pursue grand challenges. As often stated by Prof. Nozik, it is not enough to simply produce good science−the work must be useful and provide a concrete and lasting benefit to society. There is no greater challenge for scientists, at this time, than providing an affordable and sustainable energy future. Prof. Nozik’s quest led him to pursue many successful research paths. He is not afraid to propose new ideas, defend controversial concepts, or even leave behind slowly progressing research projects, even if they are being fruitful. The goal has always been to be at the forefront of the science of solar energy conversion. Prof. Nozik’s passion has been further reflected in his insistence on, and support of, employing and mentoring scientists toward the cause of a clean energy future. He has inspired and sought out many of the top scientists in the field of sustainable energy, always supporting ideas and efforts to ensure wide involvement in pursuing the challenge. Here we provide a brief overview of the status and prognosis of the many fields to which Professor Art Nozik has contributed. Bypassing the Shockley−Queisser Limit. Physics tells us that conversion of solar energy, delivered by photons, into chemical or electrical energy should be quite efficient: 66% for unconcentrated sunlight. However, all current single-junction solar conversion systems are stubbornly limited to the so-called Shockley−Queisser (SQ) limit of 33%.1 Nozik has demonstrated that this limit is not steadfast and that careful consideration of semiconductor electronic structure and charge carrier dynamics enable strategies for bypassing the SQ limit. Indeed, a substantial part of Nozik’s career has been dedicated to exceeding this limit. Photoelectrolysis and Solar Fuels. In 1977, Nozik introduced the heterotype p-n photochemical diode (Figure 1), where two semiconductors of different optical bandgaps could be combined into a monolithic structure to absorb two different parts of the solar spectrum. The band alignment of the two semiconductors and an appropriate recombination layer results in addition of the energy from photons in each region of the solar spectrum so as to achieve sufficient chemical potential for the electrons and holes to electrochemically split water. This © XXXX American Chemical Society
Figure 1. Schematic of photochemical diode energy levels. Two semiconductor absorber layers (a p-type and an n-type) are connected by a recombination layer. Blue photons from the impinging solar spectrum are absorbed in the top layer producing electron−hole pairs, and the electrons make their way to the top and reduce water to produce hydrogen while the holes move to the recombination layer. Red photons are absorbed in the bottom layer and produce electrons that move to the recombination layer and recombine with holes from the top layer. Holes in the bottom layer oxidize water to produce O2 at the surface of the metal anode.
system, similar to that of tandem solar cells, was inspired by the z-scheme of natural photosynthesis and represents Nozik’s earliest attempt at breaking the SQ limit.2 In natural photosynthesis the two bandgaps are equal; therefore, the resulting efficiency does not exceed the SQ limit. In the photochemical diode, the two bandgaps can be different, and the thermodynamically allowed conversion efficiency increases to 40% with zero overpotential. While there is a mature and growing worldwide photovoltaics industry, the same cannot be said for a solar fuels industry. Indeed, artificial photosynthetic systems that directly produce fuel from sunlight still remain in the proof-of-concept stage, yet Nozik’s photochemical diode remains a common construct furthered by many in the field. Hot Carriers. In 1982, Ross and Nozik demonstrated that if hot carriers could be utilized, the SQ limit could be bypassed in a single junction architecture. Hot carriers are those electrons and holes that are produced when the photon energy exceeds that of the semiconductor absorber band gap. The excess energy is typically lost via fast and efficient electron−phonon coupling. To slow cooling, carriers and phonons need to be decoupled.3 Received: June 14, 2016 Accepted: June 30, 2016
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conserve energy during the multiplication process integer multiples of the band gap are required to generate additional electron hole pairs, while in the hot-carrier cell the total excess energy can, in principle, be used to produce a photochemical potential difference between the anode and cathode. The first report of exciton multiplication in QDs presented by Schaller and Klimov in 2004 for PbSe QDs reported an excitation energy threshold for the efficient formation of two excitons per photon at three times the band gap (3Eg).7 This onset is already lower than what is required by momentum conservation considerations in bulk PbSe (∼4Eg) and ∼2-fold lower than what was reported for impact ionization in bulk PbSe (∼6Eg)). Reports confirming efficient MEG in PbSe QDs quickly followed. These initial reports of MEG in QDs motivated intense interest in producing solar cells that incorporated QDs in a photovoltaic cell such that multiple excitons could be separated and collected as photocurrent. Rapid progress has seen QD solar cells, where the QDs retain their useful quantum size properties, achieve power conversion efficiencies (PCEs) above 11% in a short period of time. In 2011, the National Renewable Energy Laboratory (NREL) team demonstrated that multiple electrons per incident photon circulate in a QD solar cell when the photon energy exceeds ∼3.4 eV, proving that one of the tenets of the SQ limit can be bypassed. Singlet Fission in Molecules. The 2006 Hanna and Nozik article also introduced the concept that singlet fission, the molecular equivalent of MEG, could be used in simple tandem schemes to produce up to 46% power conversion efficiencies.6 Like MEG, singlet fission had been known for several decades,8 but the process had seen very little interest in recent times. Nozik suggested that an ordinary “chromophore” that produces one singlet per photon could be paired with a singlet fission chromophore that produces two triplets but that is transparent up to the energy at which the singlet absorbs. In addition, the excited singlet from which the fission occurs is typically a priori long-lived, obviating the need for slowing cooling and enabling perfect efficiency at the energy conservation limit. These unique insights about the potential practicality of using the two triplet excitons resembled other efforts to break the SQ limit and inspired a new generation of researchers to take up the topic and to investigate both fundamental and applied aspects. Along with colleagues at NREL, University of Colorado Boulder, and Northwestern University, Nozik helped to develop new guidelines about what types of molecular systems could be the most useful.9 Today, the field encompasses a large swath of research in molecular photophysics, including contributions from high-level theory, organic synthesis, advanced femtosecond spectroscopy, and device design and fabrication. In particular, the connection that Nozik championed, between the fundamental singlet fission process and practical utilization, has gained the most attention in recent years10 and remains one of the greatest challenges in the field. The Future of Next-Generation Photoconversion. Solar energy conversion is currently at an interesting crossroads. On one hand, the current pace of material discovery and characterization is incredibly high and many of the materials have properties that are well suited for efficient solar conversion. On the other hand, the price for silicon, the dominant semiconductor in the photovoltaics sector, is at an all-time low. This juxtaposition implies that new materials face a difficult cost− benefit challenge to make the transition from the laboratory into commercial photovoltaic modules. Therefore, in order for
Hot-phonon bottlenecks, phonon bottlenecks, multiple exciton generation (MEG), singlet fission, upconversion, quantum wells, quantum dots, superlattices, and selective energy contacts are just some of the terms that are better known and studied because they could help to achieve the Ross−Nozik limit. Each one of these concepts describes processes or materials that can help to enable one of two simple outcomes for a photoconversion device: (1) enhanced photovoltage by capturing hot carriers before they cool, or (2) enhanced photocurrent by utilizing the excess energy of hot carriers to produce additional carriers. In many cases, clever manipulation of the semiconductor electronic structure via quantum confinement, communication between individual nanoscale semiconductors, and dynamic processes within the nanoscale semiconductors are employed to achieve these goals. Quantum-Conf ined Nanostructures. Concepts that can achieve the difficult feat of slowing hot-carrier cooling first evolved around the idea of quantum confinement. Such quantum confinement effects were investigated in the space-charge region of heavily doped semiconductor/liquid interfaces (quantum wells or quantum films). Highly engineered multiple quantum well structures were then envisioned and produced via metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and other techniques. Concepts to transport carriers through these wells were needed, and the concept of superlattices was explored. Slowed carrier cooling was demonstrated under concentrated light in multiple quantum well structures of GaAs/AlGaAs via a hot-phonon bottleneck;4 however, realizing hot-carrier solar cells was hampered by the need for selective energy contacts. Quantum-well structures exhibit quantum confinement in only one dimension and fast, efficient electron phonon interactions can still occur in the other two dimensions. Quantum dots (QDs), on the other hand, experience quantum confinement in all three dimensions and thus could experience a full phonon bottleneck. However, there are other cooling processes, such as Auger-cooling and interactions with ligands, solution, etc., that become important for nanoscale systems which allow for the full phonon bottleneck to be bypassed. Understanding each of these mechanisms and developing systems to manipulate their rates can result in slowed cooling. The vast potential for manipulating carrier relaxation channels in quantum-confined systems through nanostructuring should allow for significant slowing of hot-carrier relaxation rates. Multiple Exciton Generation. In 2002, Nozik realized that a different process could be accentuated in quantum-confined systems.5 Because the Coulomb interaction is enhanced in such confined systems and crystal momentum is no longer a good quantum number, resulting in hastened Auger processes, the process of exciton multiplication should also be enhanced. Multiple exciton generation in quantum dots is the equivalent of impact ionization in bulk semiconductors and describes a process whereby hot carriers generate additional carriers during their relaxation, rather than cool via generating heat. In other words, electron−electron interactions beat the electron− phonon interaction. In 2006, Nozik calculated that by making use of MEG, one could bypass the SQ limit and approach power conversion efficiencies of 44%.6 Under concentrated light, this limit could be pushed up to 88% (due to the reduced entropy and reradiation losses and thus ability to use lower bandgaps with larger multiplication factors). The efficiency is lower in the MEG cell compared to a hot-carrier cell (at 1 sun) because to 345
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ACS Energy Letters new materials to make a significant impact they should not only have cheap processing costs but also have the potential to have efficiencies that are greater than the SQ limit. This can be achieved through the concepts of hot-carrier utilization discussed above or through cheap multijunction architectures. Current costs are not yet low enough to guarantee substantial uptake of PV by consumers because the fossil fuel infrastructure is huge and entrenched. Furthermore, as photovoltaics penetrate the market the profit per unit energy decreases and thus the system costs must also continue to decrease providing ample incentive to continue pursuing low-cost solutions. There are two ways to drive down the cost of solar panels (measured in dollars per peak watt ($/Wp)): reduce module cost ($/m2) or increase module efficiency (Wp/m2) (Figure 2). Recent
Figure 3. Chemical modification of quantum-confined nanoscale systems allow for a great degree of freedom in approaching solar energy conversion. The effect of semiconductor dimensionality, aspect ratio, surface chemistry, and unique nanoheterostructures are all important considerations for improving MEG yields.
in Nozik’s footsteps, researchers at NREL discovered an efficient hot-phonon bottleneck in films of the popular methylammonium lead iodide perovskite solar material.13 The observed hot-phonon bottleneck was as efficient as that observed in the highly engineered multiple quantum wells studied by Nozik. Efforts toward a better understanding and optimizing this effect are underway in both three-dimensional bulk perovskite samples, as well as perovskites with reduced dimensionality. Whereas MEG in quantum dots shows higher efficiencies than in bulk semiconductors, the efficiencies are not yet high enough to make a large impact on solar energy conversion. Going forward, the effect of semiconductor dimensionality, aspect ratio, and surface chemistry are important considerations for improving MEG yields. Promising avenues of research include the study of novel heterostructured quantum dots, quantum rods and wires, and two-dimensional semiconductors. These studies provide an opportunity to gain a better understanding of how to control carrier relaxation processes so as to greatly enhance the MEG efficiency (Figure 3). Interest in MEG has spawned considerable activity in fabricating quantum dot solar cells, with a handful of reports successfully demonstrating MEG in devices.14−17 In these devices, MEG is realized at appreciable yields only in the blue and ultraviolet regions of the spectrum, driving researchers to work on ways to shift MEG-active solar capture to longer wavelengths. Charge transport within ordered and disordered QD arrays is still not fully understood nor optimized. Therefore, significant research remains to understand how to electronically couple quantum dots and move charge over long distances through a random array of quantum dots. Efforts are underway to dramatically improve ordering in quantum dot arrays with the intriguing potential for miniband formation that is realized in highly ordered one-dimensional superlattices. The ultimate legacy of the renewed interest in singlet fission, aside from the more detailed understanding of the process itself, may be its proliferation into a variety of solar photoconversion schemes. Coupling singlet fission molecules with bulk semiconductors (e.g., Si), semiconductor quantum dots (e.g., PbS), nanostructured oxide supports, and polymers is being done routinely and with intriguing results. Moreover, the large variety of new singlet fission molecules being discovered will add much-needed versatility to these schemes in terms of their light absorption capabilities and energy level alignment with charge or energy acceptors. There are high hopes that properly designed systems can bring to utility the two long-lived triplets born from singlet fission. With the growing contribution of renewable technologies to the energy mix, there is increasing focus on the need for energy storage processes to offset the intermittency of renewable
Figure 2. Module power conversion efficiency versus module areal costs yield the cost per peak watt (in $/Wp).12 Constant $/Wp are shown as the dashed black lines and can be decreased by increasing module efficiency at constant module cost. The $/kWh (system energy cost) includes the balance of system cost, which is assumed to be equal to the module cost here, cloud coverage, the position of the sun in the sky as it changes during the day, diurnal cycles, depreciation of the PV module, and maintenance costs. The light blue line represents the current laboratory record efficiency for bulk crystal silicon, while the dark blue horizontal line is the SQ limit for single-junction devices. Third-generation device concepts increase the limiting efficiency (the limit for MEG is indicated as the horizontal green line). The thermodynamic limit at 1 sun is shown as the red line at 67% and can be reached by an infinite stack of p−n junctions. Reprinted with permission from ref 12. Copyright 2014 Macmillian Publishers Ltd.
detailed analyses suggest that increasing module efficiency has a larger impact on reducing dollars per peak watt than reducing module cost.11 Indeed, increasing module efficiency beyond the SQ limit has a particularly dramatic effect on reducing dollars per peak watt, especially for low-cost modules, underscoring the importance of Nozik’s quest for breaking the SQ limit. Thus, we believe that third-generation solar photoconversion concepts, devices, and demonstrations should remain an enticing scientific endeavor. The stage is set for the continued exploration of novel nanoscale materials (Figure 3) that offer mechanistic advantages for breaking the SQ limit using their unique electronic structures. Within this realm, hot carriers in semiconductors remain an active area of research, with efforts toward understanding hot carriers in plasmonic structures and semiconductor/plasmonic heterostructures. Recently, following 346
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sources, such as wind and solar. In the United States, two-thirds of energy use is derived from chemical fuels (transportation and buildings) while only one-third of energy is consumed as electricity. The solar fuels concept is an enticing scientific endeavor that achieves both the harvesting of solar energy and producing a stored chemical fuel directly. As a result, there remains a great desire to increase the solar-to-fuels efficiency and stability of semiconductor interfaces for use in the photoelectrochemical production of fuels. While most efforts are focused on splitting water to produce H2 and O2, there are other important photochemical reactions that are receiving attention, such as reducing CO2 to form various hydrocarbons or alcohols, such as CH4 or CH3OH, or reducing N2 to form NH3. The synthesis and characterization of cost-effective catalysts remains a critical component of future research, with particular emphasis on mechanistic understanding of catalytic cycles. A number of interesting molecular and nonprecious metal catalysts have been explored in recent years, and new catalysts will likely be developed in the years to come. Science is ultimately a highly creative and imaginative endeavor with immense professional and personal satisfaction. Prof. Nozik demonstrated that creative science can entice a generation in the pursuit of large societal goals. While there is not yet available commercial hot-carrier solar cells, MEG solar cells, or photochemical diodes, a number of research fields from nanoscience and semiconductor interfaces to catalysts and organic semiconductorshave benefitted greatly from the impact of the work derived in the pursuit of these scientific goals. Since the 1970s there has been an enormous advance in the understanding, utilization, and appreciation of solar energy conversion in part due to the efforts of scientists such as Prof. Nozik. The scientific ideas and understanding brought forth, fought for, and championed by Prof. Nozik will continue to entice, inspire, and provide ample rewards for the current and next generation of scientists in the continued pursuit of lowcost, high-efficiency solar energy conversion.
REFERENCES
(1) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of P-N Junction Solar Cells. J. Appl. Phys. 1961, 32, 510−519. (2) Nozik, A. J. Photochemical Diodes. Appl. Phys. Lett. 1977, 30, 567−569. (3) Ross, R. T.; Nozik, A. J. Efficiency of Hot-Carrier Solar Energy Converters. J. Appl. Phys. 1982, 53, 3813−3818. (4) Nozik, A. J. Spectroscopy and Hot Electron Relaxation Dynamics in Semiconductor Quantum Wells and Quantum Dots. Annu. Rev. Phys. Chem. 2001, 52, 193−231. (5) Nozik, A. J. Quantum Dot Solar Cells. Phys. E 2002, 14, 115− 120. (6) Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100, 074510. (7) Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601. (8) Swenberg, C. E.; Stacy, W. T. Bimolecular Radiationless Transitions in Crystalline Tetracene. Chem. Phys. Lett. 1968, 2, 327−328. (9) Paci, I.; Johnson, J. C.; Chen, X.; Rana, G.; Popovic, D.; David, D. E.; Nozik, A. J.; Ratner, M. A.; Michl, J. Singlet Fission for DyeSensitized Solar Cells: Can a Suitable Sensitizer Be Found? J. Am. Chem. Soc. 2006, 128, 16546−16553. (10) Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External Quantum Efficiency Above 100% in a Singlet-ExcitonFission-Based Organic Photovoltaic Cell. Science 2013, 340, 334−337. (11) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, aad1920. (12) Beard, M. C.; Luther, J. M.; Nozik, A. J. The Promise and Challenge of Nanostructured Solar Cells. Nat. Nanotechnol. 2014, 9, 951−954. (13) Yang, Y.; Ostrowski, D. P.; France, R. M.; Zhu, K.; van de Lagemaat, J.; Luther, J. M.; Beard, M. C. Observation of a Hot-Phonon Bottleneck in Lead-Iodide Perovskites. Nat. Photonics 2015, 10, 53− 59. (14) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science 2011, 334, 1530−1533. (15) Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63− 66. (16) Böhm, M. L.; Jellicoe, T. C.; Tabachnyk, M.; Davis, N. J. L. K.; Wisnivesky-Rocca-Rivarola, F.; Ducati, C.; Ehrler, B.; Bakulin, A. A.; Greenham, N. C. Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120%. Nano Lett. 2015, 15, 7987−7993. (17) Davis, N. J. L. K.; Böhm, M. L.; Tabachnyk, M.; WisniveskyRocca-Rivarola, F.; Jellicoe, T. C.; Ducati, C.; Ehrler, B.; Greenham, N. C. Multiple-Exciton Generation in Lead Selenide Nanorod Solar Cells with External Quantum Efficiencies Exceeding 120%. Nat. Commun. 2015, 6, 8259.
Matthew C. Beard† Jeffrey L. Blackburn† Justin C. Johnson† Garry Rumbles*,†,§ †
National Renewable Energy Laboratory, Golden, Colorado 80401, United States § Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the many speakers who weathered a Colorado snowstorm to attend and present at the symposium: Mildred Dresselhaus, Casey Hynes, David Jonas, Prashant Kamat, Nate Lewis, Tim Lian, Efrat Lifshitz, Tom Meyer, Bruce Parkinson, Mark Ratner, Shozo Yanagida, Xiaoyang Zhu, and Alex Zunger. We also acknowledge funding from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through the Solar Photochemistry program Contract DE-AC36-08GO28308 to the National Renewable Energy Laboratory. 347
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